1. Field of the Invention
This invention relates to a sheet feeding device for feeding paper by applying a predetermined moving force to both edges thereof.
2. Description of the Related Art
In a printer for a computer, word processor, plotter or the like, a continuous form S as shown in FIG. 7 is often used for printing data such as characters and graphics. Feed holes S1 are provided at a predetermined pitch along the edges of the paper to be fed. A sheet feeding device 1 feeds such paper S while engaging with the feed holes S1 so as to apply a feeding force to the paper.
Usually a pair of the sheet feeding devices are disposed along the edges of the paper. In each of the sheet feeding devices 1, an endless belt 2 has a plurality of pins 3 projecting outwardly from its outer surface. These pins 3 engage with the feed holes S1 on the paper. The endless belt 2 is trained over drive sprocket wheels, not shown. When the endless belt 2 is driven by the sprocket wheel, the pins 3, on the straight portions of the endless belt 2, engage with feed holes S1 on the paper so as to apply the feeding force to the paper S.
The sheet feeding device 1 includes an openable paper presser plate 4. The paper presser plate 4 is usually kept closed by spring means, not shown. With the sheet feeding device 1 which is shown at the lower part of FIG. 7, the closed paper presser plate 4 presses the paper S toward the endless belt 2 so that the pins 3 engage with the feed holes S1 properly, which moves the paper regularly without disengagement.
To detach the paper from the sheet feeding device 1, the paper presser plate 4 is opened against the force of the spring means.
To assure normal sheet feeding, the paper presser plate 4 is required to apply a relatively large pressure to the paper so as to position the paper properly. Therefore, spring means having a relatively large spring constant is usually used to keep the paper presser plate 4 closed. However when the paper presser plate 4 is always exposed to a relatively large stress because of the force applied by the spring means, the paper presser plate 4 is often susceptible to creep. The paper presser plate 4 suffering from creep causes the clearance to vary between the paper and itself, thereby adversely affecting the sheet feeding function of the sheet feeding device 1.
To cope with such an inconvenience, it has been proposed to make the paper presser plate 4 from a material having a large Young's modulus such as polycarbonate, or materials containing a large amount of glass. However these measures lead to increased material cost, and it has been required to use inexpensive materials.